Abstract
1. A variant, denoted by delta r, of the extrachromosomal element delta (delta b) in Drosophila melanogaster was examined. The variant was carried by certain second chromosome lines, symbolized S-Cy, S-20, and SD-5 (Sandler, Hiraizumi and Sandler, 1959).
2. These three chromosomes were inviable when heterozygous for any of the other two in matings employing females of a line maintained at 25°C, or in matings employing daughters from males of these lines and females of an insensitive chromosome line (carrying no delta). Thus, delta r appeared to be transmitted biparentally at 25°C.
3. However, the heterozygous viabilities were normal when females of a line maintained at 18°C were used for matings. It is most likely that the multiplication of delta r may be delayed at 18°C.
4. Male and female heterozygotes for S-Cy and either S-20 or SD-5 produced only Curly phenotyped progeny in the presence of an appreciable amount of delta r.
5. A working hypothesis was presented that delta may be a copy of a chromosomal gene or genes, or of a certain agent which integrates inseparably into the chromosome of the fly.
